Elongate ultraviolet light source

ABSTRACT

It is desirable to be able to provide evenly illuminated, long lasting, relatively high power ultraviolet radiation for manufacturing processes such as sterilisation and ink or adhesive curing. A generally rigid waveguide ( 2 ) having a slot ( 8 ) formed in one of its short sides ( 4 ) (or optionally one of its long sides ( 6 )) may have an ultraviolet energisable elongate lamp ( 10 ) inserted therein. When the waveguide is coupled to a source of microwave energy, the slot radiates and the radiated energy is coupled almost exclusively into the lamp ( 10 ). With suitable choices of slot widths this provides even illumination with minimal microwave leakage.

[0001] This invention relates to a coupler for coupling microwave energyinto an elongate microwave energisable lamp and also to an elongateultraviolet light source.

[0002] It is well known to generate ultraviolet light using a microwaveenergisable light source. Such light sources are described, for example,in GB-A-2336240 and typically comprise an ultraviolet-transparentenvelope (typically formed from quartz) which contains a pressurised gasfill (typically of mercury and a noble gas such as argon) which whenenergised at microwave frequencies emits light through the envelopewalls from the plasma gas fill.

[0003] As has been noted in the prior art mentioned above (and the priorart discussed in the introduction thereto) there are two significantproblems which must be overcome in order to make practical use of suchmicrowave energisable lamps.

[0004] The first of these problems is that of microwave leakage.Generally speaking, microwave radiation is hazardous and therefore it isnecessary to ensure that the microwave energy used to energise the bulbis contained. This, however, is usually in direct conflict with the needto allow radiation of the ultraviolet energy.

[0005] The second problem is that of even illumination of the quartzenvelope. This is particularly important for adhesive and paint curingapplications in which is undesirable to over or under expose adjacentportions of the paint or adhesive. It may also be critical in germicidalapplications although in practice, over exposure of articles toultraviolet radiation for germicidal applications is not as critical asit is for curing applications.

[0006] The problem of even illumination becomes particularly acute whenit is desired to illuminate over a large area. For example for areashaving a minimum dimension of 150 mm or more

[0007] It is therefore an object of the present invention to provide anultraviolet light source which provides relatively even illumination atrelatively high powers over a potentially large area, for example,having a minimum dimension of ^(2λ)/₃ where λ is the microwavewavelength (which gives approximately 80 mm for a 2.45 GH_(z) microwavesource).

[0008] In accordance with a first aspect of the invention there isprovided an elongate ultraviolet light source comprising an elongatemicrowave energisable lamp and a generally rigid waveguide having agenerally rectangular cross section and four generally planar, elongatewalls, one of the walls defining a slot which passes through the entirethickness of the wall, the bulb being partially inserted into or laidover the slot and the waveguide being couplable to a source of microwaveenergy such as a magnetron.

[0009] This construction as explained below, provides a relatively highpower elongate light source which may, for example, be placed over aconveyor belt web. Thus continuous sterilisation or curing or articlespassing beneath the light source on the web may be achieved. If, forexample, the length of the lamp is 150 mm, then it will be noted thatarticles of width 150 mm at any desired length may be irradiated withultraviolet radiation.

[0010] In accordance with a second aspect of the invention, there isprovided a coupler according to claim 2, wherein the waveguide walls areof differing widths and comprise a pair of wide wall and a pair ofnarrow walls, and wherein the slot is defined in one of the narrowwalls.

[0011] Embodiments of the invention will now be described by way ofexample with reference to the drawings in which:-

[0012]FIG. 1 is a cross-sectional view of a wave guide and microwaveenergisable lamp combination;

[0013]FIG. 2 is a side elevation of the lamp and waveguide combinationof FIG. 1;

[0014]FIG. 3 is a schematic perspective view of a waveguide coupler inaccordance with the invention;

[0015]FIG. 4A is a plan view of the waveguide coupler of FIG. 3 with afirst preferred slot arrangement;

[0016]FIG. 4B is a plan view of the waveguide coupler of FIG. 3 with asecond preferred slot arrangement;

[0017]FIG. 4C is a plan view of the waveguide coupler of FIG. 3 with athird preferred slot arrangement;

[0018]FIG. 5 is a plan view of the waveguide coupler with a bulb laidthereon; and

[0019]FIG. 6 is a cross-sectional view of the waveguide of FIG. 1 with areflector.

[0020] With reference to FIG. 1, a waveguide 2 is formed from agenerally rigid and electrically conductive material such as stainlesssteel. The dimensions of the waveguide are tuned to the desiredfrequency using conventional transmission line calculations. In thisexample the desired frequency is the common microwave frequency of 2.45GHz. Other frequencies may be used consistent with the desired spectraloutput of the lamp.

[0021] As is well known, rigid waveguides of the form shown in FIG. 1have a generally rectangular configuration having a pair of short sides4 and a pair of long sides 6. With reference also to FIGS. 2 and 3, thewaveguide has a slot 8 formed in one of the sides or walls of thewaveguide 2. The drawings show the slot shown in the long sides 6. It isequally probable and perhaps more likely (depending on the standing wavepatterns within the waveguide 2) that the slot be formed in the shortsides 4

[0022] With particular reference to FIG. 2, an elongate microwaveenergisable lamp 10 is inserted into the slot and is a close mechanicalfit with the edges 12 of the slot.

[0023] By cutting a slot in the waveguide, the energy normally containedwithin the waveguide is caused to radiate through the slot 8. However,by inserting the lamp 10 partially into the slot as shown, for example,in FIG. 2, the energy is caused to energise the lamp 10 and does notleak from the waveguide or lamp since the close mechanical fit betweenthe lamp 10 and the waveguide prevents leakage around the lamp andradiation entering the lamp is attenuated to insignificant levels byvirtue of its conversion into ultraviolet light and heat by the lamp.

[0024] In practice, the waveguide will be fed with microwave energy fromone end. If the slot were to have uniform width and the lamp 10 wereinserted to be entirely parallel with the waveguide wall containing theslot, it is found that the illumination intensity reduces with distancefrom the end of the waveguide into which microwave energy is coupled.Several ways of overcoming this problem and equalising the illuminationare now described.

[0025] Firstly, with reference again to FIG. 2, one option is to use aslot 8′ of the form shown in FIG. 4A. The slot widens with distance fromthe fed end of the waveguide 14 so that (using a bulb having a generallyuniform diameter and circular cross-section) the bulb is caused togently incline into the waveguide as shown in FIG. 2. It will be notedthat the gap 16 shown in FIG. 2 is greatly exaggerated for illustrativepurposes. In practice this gap will be much smaller to prevent leakageof microwave radiation.

[0026] The widening of the slot has two effects. Firstly, it allows thebulb to be inclined into the waveguide as shown in FIG. 2 whichincreases the coupling of energy into the portion of the bulb which isinserted further into the waveguide wall. Secondly, the width of theslot directly affects the intensity of radiation of microwave energyfrom the waveguide along the length of the slot. Generally speaking, awider slot radiates more energy. Thus, a combination of the bulb beinginserted further into the waveguide and the radiation intensity beingincreased is used to compensate for a reduction in intensity ofultraviolet light input with distance from the coupled end of thewaveguide 14.

[0027]FIG. 4B shows a slot 8″ having a uniform slot width which may beacceptable in applications where variations in light intensity areacceptable, or for example, in applications in which the dimensions ofthe bulb are not uniform.

[0028]FIG. 4C shows a further embodiment in which a slot 8′″ is formedwith an exponential variation in width along its length. Thisillustrates that the slot need not have uniform variations of its widthalong its length and indeed may have notches and other features in orderto compensate for small variations in intensity along the length of thebulb.

[0029] With reference to FIG. 5, a plan view of a waveguide (using theslot shape of FIG. 4a as an example) is shown. A bulb 15 is shownoverlying the slot. In this case, the bulb substantially does not enterthe slot 8′ but is supported by the upper surface 16 of the waveguide.

[0030] With reference to FIG. 6, a reflector, preferably a focussingreflector, (for example a parabolic reflector) 18 may be formed on theupper surface of the waveguide 2 to focus light from the bulb 10 in adesired direction. The reflector 18 may be formed integrally with thewaveguide 2 or may be formed separately and secured to the waveguide 2in a separate operation.

[0031] It will be noted that it is relatively easy to machinecomplicated shapes into sheet metal material as is used for waveguideconstruction. It is easier thereby to compensate for variations inintensity using variations in slot width than by attempting to vary theconstruction of the quartz envelope of the microwave energisable lamp.This is a significant advantage over prior art constructions.

[0032] As discussed above, the construction may be inverted (relative tothat shown in FIG. 2) and held above a conveyor belt web in order toilluminate the web with ultraviolet radiation. Similarly, additionalunits may be placed vertically to illuminate the sides of relativelytall articles passing along the conveyor web.

[0033] Depending on the relative power levels and the length of the slot8, it is possible that some microwave energy will not be absorbed by thelamp 10. Since microwave energy in a waveguide may be viewed as atravelling wave, it will be noted that energy not absorbed in the slotis liable to be reflected back along the slot and the waveguide towardsthe source of microwave radiation. This is undesirable if suchreflections are at high levels since it tends to disrupt the standingwave patterns within the waveguide and thereby disrupt illumination ofthe lamp 10 resulting in uneven illumination typically athalf-wavelength intervals. Therefore, in appropriate applications, thedistal end of the slot (marked 18 in FIG. 4A for example) may befurnished with “lossy” material which attenuates energy at microwavefrequencies and thereby absorbs surplus energy rather than allowing itto become reflected by the end of the slot.

1. A coupler for coupling microwave energy into an elongate microwaveenergisable lamp comprising a generally rigid waveguide having agenerally rectangular cross section and four generally planar, elongatewalls, one of the walls defining a slot which passes through the entirethickness of the wall.
 2. A coupler according to claim 1, wherein thewaveguide walls are of differing widths and comprise a pair of widewalls and a pair of narrow walls, and wherein the slot is defined in oneof the narrow walls.
 3. A coupler according to claim 1, wherein the slotis of non-uniform width along its length.
 4. A coupler according toclaim 3, wherein the slot width varies exponentially with length.
 5. Acoupler according to claim 3 wherein a first end of the coupler isarranged to receive microwave power form a power source such as amagnetron and wherein the slot is arranged to become wide withincreasing distance from the first end.
 6. A coupler according to claim5, wherein the slot width varies exponentially with length.
 7. A coupleraccording to claim 2, wherein the slot is of non-uniform width along itslength.
 8. A coupler according to claim 7, wherein the slot width variesexponentially with length.
 9. A coupler according to claim 7 wherein afirst end of the coupler is arranged to receive microwave power form apower source such as a magnetron and wherein the slot is arranged tobecome wide with increasing distance from the first end.
 10. A coupleraccording to claim 9, wherein the slot width varies exponentially withlength.
 11. A coupler according to claim 1 including a focussingreflector positioned adjacent the slot.
 12. An elongate ultravioletlight source comprising an elongate microwave energisable lamp and agenerally rigid waveguide having a generally rectangular cross sectionand four generally planar, elongate walls, one of the walls defining aslot which passes through the entire thickness of the wall, the bulbbeing partially inserted into or laid over the slot and the waveguidebeing couplable to a source of microwave energy such as a magnetron.